The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to reduce the running costs of the base station. One option to do this is to implement a radio system as an antenna-embedded radio forming an active antenna array. Many of the components of the antenna-embedded radio may be implemented on one or more chips.
Multiple receive paths in the antenna-embedded radio need to be synchronised in phase, delay and amplitude of signals travelling on the receive paths. Known techniques to establish variations in the phase, delay and amplitude of signals involve the injection of a known signal, termed the sounding signal, into one or more of the receive paths and, based on the comparison of the sounding signal and the received signal, the phase, delay and amplitude variations for the signals in the receive paths can be estimated. This allows for calibration of the receive paths by generation of correction coefficients to be applied to receive signals received along the multiple receive paths.
The sounding signal can have either the same frequency in a carrier signal spectrum or be at a different frequency than the carrier signal spectrum. In the first case (frequency of the sounding signal is in the carrier signal spectrum) then it is necessary to correctly adjust the power of the sounding signal. If the power of the sounding signal is too high, then the quality of the carrier signal can be degraded. On the other hand, if the power of the sounding signal is too low, the quality of the measurements of the phase, delay and amplitude variations is too low.
If the sounding signal is positioned in a frequency spectrum different from the carrier signal spectrum, then the frequency and phase response of the analogue receive filters in the receive paths can be slightly different at the different frequencies. This implies that the measurement results for the phase, delay and amplitude of the signals measured at the frequency of the sounding signal may be slightly different than the measurement results for the phase, delay and amplitude of the signals measured at the frequency of the carrier signal. In addition, it is necessary to ensure that the frequency of the sounding signal is different than any of the frequencies of the other carrier signals which might be measured at the antenna embedded radio. There is also a risk that blockers in the antenna embedded radio may block certain frequency bands and thus affect the quality of the error measurement. Finally the sounding signal might be unintentionally transmitted from a receive antenna and then be detectable at a receive port of another (unconnected) receiver, which might violate regulations.
A further known solution is to use a wide-band spectrum, for example a spread spectrum sounding signal, which is close to or below the noise floor of the carrier signals. In order to avoid the blockers, an extremely long sounding signal spreading code is necessary, in order to have sufficient processing gain.